Mine methane is defined as a gas of predominately methane (CH4) with lower concentrations of non-methane hydrocarbons, nitrogen, ammonia, carbon dioxide, and other trace gases. Mine methane is released into the mine atmosphere during the mining of trona.
Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). A vast deposit of the mineral trona is found in southwestern Wyoming near Green River. This deposit includes layers of trona and mixed trona and halite (rock salt or NaCl) which covers approximately 2,600 km2. The major trona beds range in size from less than 428 km2 to at least 1,870 km2. By conservative estimates, these major trona beds contain about 75 billion metric tons of ore. The different beds overlap each other and are separated by layers of shale and marlstone. The quality of the trona varies depending on its particular location in the stratum.
A typical analysis of the trona ore mined in Green River is as follows:
TABLE 1ConstituentWeight PercentNa2CO343.2NaHCO333.7H2O (crystalline and free moisture)15.6NaCl0.1Insolubles7.3
The sodium sesquicarbonate found in trona ore is a complex salt that is soluble in water. The mined trona ore is processed generally in a surface refinery to remove the insoluble material, organic matter and other impurities to recover the valuable alkali contained in the trona.
The most valuable alkali produced from trona is sodium carbonate. Sodium carbonate is one of the largest volume alkaline commodities produced in the United States. In 2007, trona-based sodium carbonate from Wyoming comprised about 91% of total U.S. soda ash production. Sodium carbonate finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
The trona deposits found in Southwestern Wyoming are formed in multiple beds in the Wilkins Peak Member of the Eocene Green River Formation at depths ranging from 240 to 910 meters (800-3000 feet). The Wyoming trona deposits are evaporites that form substantially horizontal beds. The beds vary greatly in thickness, from about 0.3 meter to about 5 meters (about 1-16 feet). An underground formation comprising a trona bed generally further comprises a methane-bearing layer. Interbedded with trona beds are layers of methane-bearing oil shales. For example, layers of mainly weak, laminated green-grey shales and oil shale may be above a trona bed. Immediately below the trona bed may lie substantially horizontal layers of somewhat plastic oil shale. Both overlying and underlying shale layers can liberate methane during mining. It is also possible for marlstone layers to liberate entrapped methane upon fracture. The trona itself contains very little carbonaceous material and therefore liberates very little methane. According to a first known mining technique, called the room-and-pillar technique, a number of rooms are created in the underground formation, connected by an array of tunnels. Between the rooms, a series of trona pillars are left in place to support the roof of the mine rooms. The disadvantage of this technique is that the trona contained in the pillars is not mined, resulting in a loss of valuable mineral.
According to another mining technique, called the solution mining technique, the mineral is recovered by introducing a fluid from the earth's surface to dissolve the trona deposit. The solution, when enriched in dissolved trona is pumped out of the formation and treated in a surface refinery. The solution mining can create a mined-out cavity within the trona ore. From the weight of the overburden, caving of overlying trona and rock may occur, which could result in strata subsidence and fracture of underlying and/or overlying oil shales, thus liberating mine methane into the caved-in area.
According to a third well known mining technique, called the “long-wall” mining technique, the roof is supported by movable hydraulic supports as the trona is mined. After mining the trona, the supports are advanced, allowing the unsupported roof to collapse. The caved-in area comprising fallen broken rock may be referred to as “gob”. The gob formation is generally accompanied by fracture of overlying and/or underlying oil shale layers.
When utilizing a mining technique for trona which results in fracturing one or more oil shales, a significant amount of methane can be liberated from the fractured oil shale(s). This released methane can mix with the mine air, and in such event, the released methane must be diluted to safe levels in the return airways of the mine's ventilation system in order to then be exhausted to the atmosphere. The method of diluting the methane using the mine ventilation system requires that the methane concentration be diluted with air from a high level to a low level. Through dilution, the methane content passes through the explosive range of methane-air mixture (5%-15% methane in air) during the process. This passage through the explosive range causes a safety risk, and thus to avoid hazardous methane contents in mine working places and return airways, a large volume of dilution air is required. If additional methane is released, additional air for dilution is necessary. Additionally, regulations require a maximum allowable methane content in the return airways, which is generally less than 1%. The additional air requirement increases ventilation pressure which results in increased air leakage through ventilation structures and increased energy consumption.
Although these foregoing issues have been described in terms of trona mining, they also apply to any mine in which a non-combustible ore is extracted and which is capable of liberating methane during the mining of the non-combustible ore.